Calgarians were treated to a rare display of shimmering noctilucent clouds above the city on Thursday night, and took to social media to share the phenomenon.

According to NASA, clouds classified as "noctilucent" — which is Latin for "night-shining" — are at an altitude of roughly 80 kilometres.

This makes them the highest clouds in our atmosphere. Smithsonian Magazine reported in 2018 that normally, clouds form at a maximum height of four miles above the earth's surface.

Researchers told the CBC in 2019 that there are still unknowns about noctilucent clouds, but three things need to be present in order for them to form: an increase in water vapour, very cold temperatures, and particles such as meteoroid dust on which the water vapour can freeze.

The sun must also be at least six degrees below the horizon to illuminate the clouds and the icy particles within them, which creates a ghostly, shimmering effect even after dark.

When noctilucent clouds are spotted, it is usually on clear summer evenings, but the odds of seeing them might be getting better. A 2018 study suggested that an increase of water vapour in the upper atmosphere caused by human activity is also increasing the phenomenon's occurrence.